Wood and wood composite products are well known. Wood composites are in widespread use in furniture and other consumer products. Some examples of specific wood composite products are particleboard, medium density fiberboard (MDF), and oriented strandboard (OSB).
Particleboard is formed by binding small wood flakes with an adhesive, then rolling or molding a billet or sheet of the treated flakes to form a board, beam, or other product form. Randomly oriented particleboard has different mechanical properties from ordinary sawn timber wood. Wood exhibits directional mechanical properties, owing to the natural alignment of long wood fibers along the direction of the tree trunk. Its tensile strength and elastic modulus, for example, are much greater in directions parallel to the grain direction than in the cross-grain direction. In contrast, the random alignment of wood flakes in particleboard and some other wood composite products results in substantially isotropic mechanical properties. But these isotropic properties are comparable to the relatively poor mechanical properties possessed by wood in directions perpendicular to the grain. In general, particleboard has exhibited a poor ability to sustain bending loads, as compared with natural timber, and thus has largely been unsatisfactory as a structural beam.
Medium density fiberboard is made similarly to particleboard, except that the flakes commonly are smaller, and are refined to release fibers before forming sheets.
Wood composite products have been developed in which wood flakes or strands are oriented in a single direction, to provide a structure more like natural wood. One example is oriented strandboard (OSB). Such products possess relatively improved strength in directions parallel to the direction of alignment.
More complex wood composite products have been developed, which have several consolidated strata in which the particles are aligned in different directions. For example, a three-stratum beam or sheet or other composite article can be made in which the outer or facing layers have their orientation parallel to the longest dimension of the composite, as in conventional wood. The interior layer has its fiber orientation perpendicular to the longest dimension of the composite. Three-stratum boards are also known in which large flakes make up the center layer and smaller flakes make up the outer layers.
Reconstituted wood particle boards may be made by various processes. One example includes pouring a slurry consisting of wood particles, water, and discontinuous high strength, high modulus fibers onto a continuously moving screen. The water is drained off leaving a wet wood particle mat on the screen. The interlaced wood particle mat is then oven dried producing a continuous sheet of fiberboard. Hot steam jets can be used to increase the density of the material to produce wet-process hardboard. This process may be used to produce hardboard, and low, medium and high density fiberboard.
Another method includes mixing sawdust, wood shavings, wood waste or veneer with an adhesive containing the discontinuous fibers and compressing the mixture under heated platens. Different profiles of wood material can be used through the depth. For example, saw dust or small wood waste materials can be placed between wood wafers and pressed under heat. Organizing the layup through the thickness allows for increased engineering properties including strength and stiffness. Molds can be used to produce complex shapes. Plywood, oriented strandboard, waferboard, particleboard, medium and high density fiberboard, and laminated veneer lumber may be made by this process.
Yet another method for manufacturing a reconstituted wood structural member comprises combining wood particles, strands, or veneers with a binder mixed with the discontinuous fibers and forcing the combination through a die under heat and pressure to produce a continuous board. The finished board can be cut to particular lengths and it can intricate profiles determined by the cross section of the die. This process may be used to produce parallel strand lumber, particle board, and variations of laminated veneered lumber.
Reconstituted wood products may also be made by combining wood chips or other type of wood waste with a wax or other type of binder with the discontinuous fibers and subjecting the combination to heat and pressure on the hot press. This dried processed board is similar to the wet processed board except that the mechanical properties are not as great due to the decreased fiber interlocking. Hardboard; high density hardboard; fiberboard; and low, medium, and high density fiberboard may be made from this process.
There are many different problems associated with wood and wood composite products. One problem is that they are dimensionally unstable after they are made. Even a stratified board with layers oriented in different directions will grow or shrink substantially in response to environmental moisture and weather conditions. This property has limited the value of wood composite members of substantial size for use in construction products.
The problem of dimensional instability can be addressed by increasing the adhesive content of the product, but at a substantially greater cost, as the adhesive composition is expensive. Another way known in the prior art to improve the dimensional moisture stability of the resulting board product is to apply a suitable wax in emulsion or molten form to the wood particle mix at the binder blender station or elsewhere in the fabrication process.
Anther problem is that of preservation against environmental factors, such as but not limited to, termites, ants (for example, carpenter ants) and other wood-destroying insects or fungi, soft rot, and mold fungi. Examples of wood-destroying fungi and soft rot and mold fungi are: Gloeophyllum trabeum, Trametes versicolor, Paxillus panuoides, Condrostereum purpurescens, Heterobasidium annosum, Bispora effusa, Stachybotrys atra, Chaetomium globosum, Trichoderma viride, Aspergillus niger, Hormiscium spec., and Stemphylium spec. Wood products are preserved using amounts of wood preservative compounds known or believed to be effective against one or more of these organisms.
Yet another problem in the art has been how to effectively incorporate a water-borne wood preservative in a wood composite product, so the preservative compounds reach the interior of the product, without also incorporating a significant amount of additional water in the product. Any excess incorporated water must be dried out, using additional energy, time, equipment, factory space, and thus money. Another problem in the art is how to incorporate a water-borne preservative system into wood composite products without causing a negative impact on panel structural properties.
U.S. Pat. No. 4,241,133 to Lund, et al. describes a wood composite containing about 5 to 12% weight of a binder and, optionally, additives, such as wax, for waterproofing and preservatives for protection against decay fungi and insects. According to Lund, dried, classified particles are introduced into a conventional blender where predetermined amounts of a binder, and optionally a wax, a preservative and other additives are applied to the particles as they are tumbled or agitated in the blender.
U.S. Pat. No. 6,569,540 to Preston et al. describes a wood composite including wood particles, a binder, at least one wood stabilizer, and optionally other ingredients. The wood stabilizer is present in an amount effective to reduce the swelling value of the wood composite to less than that of an analogous wood composite not treated with the wood stabilizer. The wood composite may be made by applying a wood stabilizer to green wood particles, preferably without an intervening drying step. While or after applying the stabilizer, a water repellant material (e.g., a wax emulsion) is applied to the wood particles. The treated wood particles are then formed into a wood composite.